A method for removing debris from pipelines with a gel plug is disclosed in U.S. Pat. No. 4,216,026. This method is an improvement over other methods used to remove debris from pipelines which include the use of scrapers, high velocity liquid flow and ordinary gel plugs. Thus, mechanical scrapers and ordinary gels tend to concentrate the debris, leaving it in thick beds along the bottom of the pipeline. In the case of high velocity liquid flow, adequate pump capacity and/or volume of fluid needed to remove the debris often are unavailable. By comparison, the method of U.S. Pat. No. 4,216,026 utilizes a Bingham plastic gel plug which is a flowable, nonthixotropic plastic composition having less moving shear stress at the wall of a pipeline than strength of adhesive bonding to the wall of the pipeline, to facilitate a peculiar type of flow which effectively entrains debris within the body of the gel plug. Thus, the Bingham plastic gel plug moves through the pipeline by a rolling or a circulating motion generating a closed toroid, the wall of the toroid adjacent the wall of the pipeline remaining relatively stationary and the center portion of the toroid moving in the flow direction, thereby entraining debris within the gel plug.
Even though the improvement over the prior art represented by the invention of U.S. Pat. No. 4,216,026 is substantial, it now has been discovered that other improvements can be made which even further increase the efficiency and effectiveness of this invention. Thus, it has been discovered that the trailing displacement fluid (normally water) employed to push a pig, scraper or separator and leading gel plug, has a tendency to by-pass forward, i.e., move past the separator and into the gel plug. This dilutes and/or otherwise destroys the debris carrying property of the gel. Accordingly, it is desirable to provide means which substantially eliminate or reduce by-pass forward of the trailing displacement fluid.